I have found something rather peculiar about rewinding live-tv. I'm using the latest Gotham RC and pvr.tvh 0.7.0 (adamsuttons git-repository). Both servers and clients are running ubuntu 14.04. Tvheadend is the latetst "beta." (3.5.XX, I think. Before the new dvb-code was merged).
I can pause, rewind and fast-forward live-tv as you'd be expecting with a "normal" tv-broadcast, just like any normal avi/mkv/mpg/whatever. However, if there is no EPG-data for the tv-program I'm currently watching, rewind is impossible! Quite funny, I'd say
Maybe you're experiencing the same phenomenon?
I'm currently at work, so I can't post logs or anything. But I've been meaning to file a bugreport. But I don't know which component "to blame" yet.
To finish, I would like to give nothing but praise for both XBMC and tvheadend, they're both awesome products.